A boiling water reactor (BWR) has nuclear fuel assemblies comprising bundles of fuel rods made of fissionable materials capable of releasing a finite number of neutrons. Neutrons are released due to fission at high speed and then moderated by the water to a lower speed at which the neutrons can produce a chain reaction of fission events. Each fuel assembly is surrounded by a fuel channel made of metal which parasitically absorbs neutrons. To minimize parasitic absorption of neutrons, fuel channels are typically fabricated from a metal alloy known as Zircaloy, which absorbs very few thermal neutrons, i.e., has a low absorption cross section. Zircaloy is an alloy of zirconium with small amounts of iron, tin and other alloy metals. In particular, Zircaloy-2 contains about 1.5% tin, 0.15% iron, 0.1% chromium, 0.05% nickel and 0.1% oxygen, whereas Zircaloy-4 contains substantially no nickel and about 0.2% iron but otherwise is similar to Zircaloy-2.
Fuel channels are hollow elongated components of generally square cross section, which may measure approximately 6 inches on each side and on the order of 14 feet in length. Typically, the channels are created by rolling strips of Zircaloy, bending the strips into U-shaped channel sections and then seam welding two U-shaped channel sections together to form a single fuel channel. Reactor control occurs typically on the exterior of such fuel channels. To optimize nuclear reactor control, the fuel channels are formed with flat sides which conform to the shapes of the control rods.
The fuel channels also serve the purpose of confining the coolant water flowing through the nuclear core to a predictable flow path around the steam-generating fuel rods. To assure predictable water flow in the reactor, there is a pressure differential between the inside and the outside of the fuel channels. The water flowing inside the fuel channel is turned into steam by the heat released during fission. The outside of the fuel channel constitutes a different pressure environment.
The planar fuel channel walls are connected by corners and tend to bulge responsive to the pressure differential. This tendency to bulge is additionally aggravated by thermal effects and also by radiation. In-reactor dimensional changes of a fuel channel are primarily a result of: 1) stress relaxation of manufacturing-induced stresses; 2) radiation-induced growth; and 3) radiation-enhanced creep. The radiation-induced growth can lead to dimensional changes such as longitudinal bow if the channel strip contains variations in crystallographic texture and resides in a region with neutron flux gradients. In particular, conventional fuel channels having a uniform crystallographic texture, i.e., f.sub.L .apprxeq.0.10 everywhere in the fuel channel, will bow in a flux gradient.
Irradiation also decreases the ductility and increases embrittlement of the Zircaloys. The magnitude of the radiation effect is partly dependent on the microstructure (or "texture") of the alloy. Because the hexagonal close-packed phase in the Zircaloys is anisotropic, different fabrication processes will yield products with different textures. Therefore, the radiation embrittlement of Zircaloy is dependent on its fabrication history.
Since in-reactor dimensional stability and corrosion resistance are important attributes of a BWR fuel channel, it is imperative that these fuel channels be manufactured to the proper dimensions and be free of geometric irregularities, such as face or side bulge, out-of-square cross section, non-parallelism of sides, longitudinal bow and twist, and the like. However, the channel creating step leaves residual manufacturing stresses which lead to geometric irregularities. Therefore, it is conventional practice to subject fuel channels to thermal sizing to eliminate these stresses.
The use of thermal sizing is well established in the fabrication of precisely dimensioned components and in various other processes. This technique takes advantage of differences in the coefficients of thermal expansion of different metals. An elongated, close-fitting mandrel having a coefficient of thermal expansion greater than that of the component to be sized is inserted into the component. This assembly of the component and mandrel is then heated to a temperature of about 1100.degree. F. in an inert atmosphere, e.g., in a vacuum or in an inert gas such as argon. As the mandrel expands at a greater rate than the component, the former plastically deforms the latter to the desired dimensions while relieving manufacturing stresses. The assembly is then cooled and the mandrel is removed. Thermal sizing techniques are disclosed in U.S. Pat. No. 4,989,433 to Harmon et al. and U.S. Pat. No. 4,604,785 to Eddens, both assigned to the assignee of this patent application, and in U.S. Pat. No. 3,986,654 to Hart et al. The contents of these patents are incorporated by reference herein.
High corrosion resistance for the Zircaloys is conventionally obtained by heating the channel material to an elevated temperature followed by fast quenching, e.g., by inductive heating and water quenching. For Zircaloy-2 the process involves quenching at an intermediate slab thickness and controlling subsequent thermal exposure during strip manufacture.
Such a heat treatment is disclosed in U.S. Pat. No. 4,238,251 to Williams et al., assigned to the assignee of this patent application and the contents of which are incorporated herein by reference. This patent discloses that in components made of zirconium-based alloys, a strong correlation exists between a particular microstructural characteristic and resistance to accelerated pustular corrosion in BWR environments. That characteristic can be produced by heating to redistribute the intermetallic particulate phase [Zr(Cr,Fe).sub.2 in Zircaloy-4 and Zr(Cr,Fe).sub.2, Zr.sub.2 (Ni,Fe) in Zircaloy-2] in a pattern which imparts the desired corrosion resistance characteristic to the metal. U.S. Pat. No. 4,238,251 teaches that the service life of a zirconium-based alloy component can be greatly increased by heating the component to initiate transformation from alpha (hexagonal close-packed) to beta (body-centered cubic) phase, and then quenching to a temperature substantially below the phase transformation temperature range. While transformation of the alpha phase to the beta phase begins at about 825.degree. C., a somewhat higher temperature, e.g., 870.degree. C., was preferred. Segregation of precipitate particles is obtained to the desired extent by quenching after only a few seconds in the transformation temperature range down to below 700.degree. C.
U.S. Pat. No. 4,238,251 further discloses that rapid cooling enhances the corrosion properties of fuel channels in service in BWRs, without degrading physical properties in general and creep strength and ductility in particular. Preferably, this cooling step involves quenching the component at a rate of at least about 20.degree. C. per second.
Thus, conventional processing techniques use thermal sizing to reduce manufacturing stress or controlled thermal treatment to impart the required corrosion resistance. Considerable effort is also invested in providing what is known in the industry as matched pairs to reduce crystallographic texture variation and minimize differential irradiation growth within channels. However, it has not been known to provide a dimensionally stable and corrosion-resistant fuel channel by heat treatment of the channel strip material, warm forming of the channel components and thermal sizing of the assembled channel.